In various pneumatic tire constructions, including but not limited to bias ply tires used for a variety of applications, as well as radial ply tires used for aircraft applications, military applications, OTR (off-the-road) applications, Passenger, and RMT (radial-medium-truck) applications, a barrier typically comprises a component layer of the tire. The barrier may constitute a single layer among the various layers assembled to form a tire, but several layers may also act in combination as a barrier.
The innerliner typically is the innermost layer or combination of layers in the assembled tire. Over the service life of a tire, the innerliner becomes susceptible to cracking or breaching at one or more points along the innerliner surface as a result of interactions with adjacent layers within the tire assembly. The innerliner is normally prepared by conventional calendaring or milling techniques to form a strip of uncured compounded rubber of appropriate width which is sometimes referred to as a gum strip. Typically, the gum strip is the first element of the tire applied to a tire building drum, over and around which the remainder of the tire is built. When the tire is cured, the innerliner becomes an integral, co-cured, part of the tire. Tire innerliners and their methods of preparation are well known to those having skill in such art.
Particularly, there is a risk that movement of a ply layer adjacent to the innerliner over the service life of the tire relative to the innerliner could cause a breaching of the innerliner layer in service, thereby permitting movement of air and water through the innerliner.
To minimize the risk of innerliner breach in this manner, a barrier layer is commonly interposed between the innerliner and ply layers. Where the ply layer is typically formulated from a high modulus rubber, and the innerliner a low modulus halobutyl rubber, the barrier layer typically is prepared using an intermediate modulus rubber. The barrier layer functions as a pad to buffer the physical contacts between the ply and innerliner layers, and to reduce stresses between these layers.
In serving this function, the barrier gauge, or thickness, must be sufficiently high to continue to provide this buffering effect during manufacturing and the service life of the tire. The desired cured barrier thickness must be maintained both during the building and shaping of the uncured barrier and tire as well as during the tire curing process. In certain tire end use applications, such as OTR, tremendous stresses can be experienced in the vicinity of the barrier layer. Because insufficient gauge of the barrier layer may result in potential damage to the innerliner and reduce the tire durability, it is important that the barrier layer in the cured tire maintain sufficient thickness in the manufacturing process to reduce the risk of damage to the innerliner in service.
One of the measures of the tendency of the barrier layer to deform or to flow in the uncured state under shaping and curing stresses is the green strength. In elastomers which possess poor green strength the yield stress which the unvulcanized elastomer exhibits during deformation is low and the stress drops off quite rapidly as the deformation continues. Unvulcanized strips or other forms of such elastomers often pull apart during building operations. Also, the gauge of the green barrier component can be reduced by shaping and curing pressure stresses. Green strength is typically quantified in terms of the stress/strain curves of the unvulcanized elastomer. Usually, the performance of a green compound (unvulcanized) is based upon two points of the stress/strain curve, namely the first peak or yield point and the ultimate or breaking tensile point. Improvement in either of these stress/strain properties indicates improved green strength.
Numerous additives and increased loading of carbon black, silica, or both in the composition have been utilized in association with various elastomeric mixture modifications to improve green strength. However, the utilization of such methods to improve green strength commonly causes unwanted results, such as reduction in component to component adhesion or the loss of flex life of the compounded elastomeric mixture. For various reasons, the incorporation of additives into elastomers in order to improve green strength has generally not proven to be completely satisfactory. Electron beam precure is a technique which has gained wide commercial acceptance as a means of improving the green strength of elastomers. For instance, electron beam precure is widely used in the tire industry to improve the green strength of elastomers utilized in building tires. However, electron beam precure techniques are costly, due in part to complex handling equipment requirements and isolation of the electron beam energy and its byproducts. Nevertheless, electron beam precure often represents the only acceptable means for providing adequate green strength to maintain dimensional stability during tire building and curing procedures without adversely affecting the desired cured component (barrier) properties like flex life.
There remains a need especially for individual barrier layers of a tire to have good green strength to maintain green and cured gauge during the tire building and curing processes without resorting to increasing the barrier gauge, with the associated weight and cost gains, and also to maintain good flexibility post-cure to achieve maximum service life.